4.12.2.1 Property

You can use build properties in most step paramaters. Please file bugs for any
parameters which do not accept properties. The simplest form is to wrap the
property name with Property, passing an optional default argument.

You can specify a default value by passing a default argument to
Property. This is normally used when the property doesn't exist,
or when the value is something Python regards as "false". The defaultWhenFalse
argument can be used to force buildbot to use the default argument only
if the parameter is not set.